Cytopathologic interpretation of ascites due to malignancy

The diagnosis of metastatic cancer in peritoneal fluid is of great importance for the patient and the attending physician. A cytopathologist's responsibility is twofold: (1) to accurately identify malignant cells; (2) to interpret tumor type and if possible the site of its origin even in the absence of complete clinical history of other clues. The difficulty in the diagnosis of metastatic neoplasms in peritoneal fluid is due to 2 factors: (1) abnormal mesothelial cells or macrophages may simulate cancer cells, or may conceal tumor cells; and (2) peritoneal fluid constitutes a natural and hitherto inadequately explored medium of cell culture, in which neoplastic cells may proliferate free of the boundaries imposed upon them by the framework of organs and tissues. Immunocytochemistry (ICC) and molecular techniques are essential to establish an accurate diagnosis. From a great many points of view malignant peritoneal fluid is suitable for continuous study of cancer cells, thus providing knowledge about biologic aspects of human solid tumors.